Generally speaking, a hydraulic excavator drive system includes a turning motor, a boom cylinder, an arm cylinder, and a bucket cylinder as hydraulic actuators. Two hydraulic pumps supply hydraulic oil to these hydraulic actuators. Usually, the supply of the hydraulic oil to the turning motor is controlled by one control valve, and the supply of the hydraulic oil to the bucket cylinder is controlled by another control valve. Meanwhile, the supply of the hydraulic oil to the boom cylinder (at least when a boom raising operation is performed) is controlled by two control valves, and the supply of the hydraulic oil to the arm cylinder is controlled by other two control valves.
For example, Patent Literature 1 discloses a hydraulic excavator drive system 100 as shown in FIG. 9. In the drive system 100, an arm main control valve 121 and a boom auxiliary control valve 132 are disposed on a first bleed line 102 extending from a first hydraulic pump 101, and an arm auxiliary control valve 122, a bucket control valve 110, and a boom main control valve 131 are disposed on a second bleed line 104 extending from a second hydraulic pump 103.
The arm main control valve 121 is connected to an arm operation valve 120 by an arm crowding pilot line 123, and the boom main control valve 131 is connected to a boom operation valve 130 by a boom raising pilot line 133. An auxiliary pilot line 124 branches off from the arm crowding pilot line 123, and connects to the arm auxiliary control valve 122. Similarly, an auxiliary pilot line 134 branches off from the boom raising pilot line 133, and connects to the boom auxiliary control valve 132. The auxiliary pilot lines 124 and 134 are provided with solenoid proportional valves 125 and 135, respectively.
Each of the solenoid proportional valves 125 and 135 outputs a pilot pressure to the auxiliary control valve (122 or 132), the pilot pressure decreasing in accordance with an increase in a pilot pressure outputted from the operation valve (120 or 130). That is, the pilot pressures outputted from the solenoid proportional valves to the auxiliary control valves are inversely proportional to the pilot pressures outputted from the operation valves to the main control valves. When a pilot pressure led to an auxiliary control valve decreases, the degree of opening of the auxiliary control valve is reduced. Patent Literature 1 describes that, owing to this configuration, when an arm crowding operation and a boom raising operation are performed concurrently, the hydraulic oil can be preferentially supplied to one of an arm cylinder 126 and a boom cylinder 136. The time when an arm crowding operation and a boom raising operation are performed concurrently means the time when the bucket is moved horizontally in a manner to bring the bucket closer to the body of the excavator.